Head in barrel rolled back (3)
I believe the answer is:
nut
'head' is the definition.
(informal term for somebody's head)
'barrel rolled back' is the wordplay.
'barrel' becomes 'tun' (tun is a kind of barrel).
'rolled back' says the letters should be written in reverse.
'tun' in reverse letter order is 'NUT'.
'in' acts as a link.
(Other definitions for nut that I've seen before include "Fanatical enthusiast" , "Fruit with hard shell" , "Hard-shelled fruit or informal eccentric" , "Hard fruit or informal lunatic" , "Hard shell of a locking device" .)